There are a couple ways to solve this. One way, and probably the easiest way, is by using proportions.
The two fractions we need to use are 12/x and 60/100 These two fractions are equal, but we don't know what the denominator of the first fraction.
We can find that out by cross multiplying. If our fractions are a/b and c/d we need to multiply a and d then b and c. Somehow, these two products are going to match up.
So, we multiply 12 and 100 to get 1200. Multiplying x and 60, we get 60x.
Since 1200=60x, we just need to divide 1200 by 60 to get x. (That equals 20 btw)
In conclusion 12 is 60% of 20.
Any questions?
